This proposal is to determine a set of structures of the translation regulator Smaug with the aim of elucidating its interaction with RNA and other proteins. In the developing Drosophila embryo, Smaug represses the translation of the protein Nanos through an interaction with a portion of Nanos mRNA termed the translation control element (TCE). The structure of a Smaug/TCE complex should illustrate specific molecular contacts which could then be used to design modifications to the protein and RNA to test the thermodynamic importance of each contact. Presently, there are few solved structures of protein-RNA complexes, and most involve the protein bound to double stranded helices of RNA. However, it has been shown that Smaug likely binds to the single stranded loop at the end of the TCE hairpin. Because the RNA binding domain of Smaug shows no similar to any other known RNA binding motif, a Smaug/TCE structure promises to reveal a noel protein/RNA interface. An attempt will also be made to solve the structure of a complex between Smaug and Oskar, a protein that acts to derepress Nanos translation through an interaction with Smaug. Since the same domain of Smaug interacts with both RNA and Oskar, it will be interesting to see how the two binding sites differ and, in particular, whether the two sites overlap. Separate binding sites will open up the possibility of solving the structure of an Oskar/Smaug TCE complex. Translation regulation is one of the last steps in the control of gene expression, and is an important checkpoint in the control of cell differentiation A basic understanding of the protein/RNA interactions that are involved will increase our understanding of the various mechanisms of diseases such as cancer and birth defects, and lead to the development of practical therapies.